1. Field of the Invention
The present invention relates generally to a fluid-filled vibration damping device adapted to provide a vibration damping or isolating effect on the basis of flows or resonance of a non-compressible fluid which is forced to flow through an orifice passage. More particularly, the present invention is concerned with such a fluid-filled vibration damping device which is capable of changing its damping characteristics by selectively enabling and disabling a first and a second orifice passages which are tuned to different frequency bands, depending upon the type of input vibrations having different frequencies, for thereby exhibiting an excellent vibration damping or isolating effect with respect to the input vibrations over a wide frequency range.
2. Description of the Related Art
As one type of a vibration damper interposed between two members of a vibration system, there is known a fluid-filled vibration damping device which is capable of changing its damping characteristics on the basis of flows or resonance of a non-compressible fluid which is forced to flow through a first or second orifice passages which are tuned to different frequency bands, by selectively operating the first and second orifice passages, depending upon the type of input vibrations having different frequencies. Such a fluid-filled vibration damping device, as disclosed in U.S. Pat. No. 5,642,873 and JP-A-9-280304, for example, includes a first mounting member and a second mounting member having a generally cylindrical shape, which are spaced apart from each other by a suitable distance and elastically connected to each other by an elastic body such that one of axially opposite open end of the second mounting member is fluid-tightly closed by the elastic body. The other open end of the second mounting member is fluid-tightly closed by a flexible diaphragm. Between the elastic body and the flexible diaphragm, there is disposed a partition member which is fixedly supported by the second mounting member. A pressure-receiving chamber which is partially defined by the elastic body is formed on one side of the partition member, while a variable-volume equilibrium chamber which is partially defined by a flexible diaphragm is formed on the other side of the partition member. These pressure-receiving and equilibrium chambers are filled with a suitable non-compressible fluid, and communicate with each other by a first orifice passage and a second orifice passage which is tuned to a frequency band higher than the frequency band to which the first orifice passage is tuned, which first and second orifice passages are formed in the partition member. An actuator is disposed on one side of the flexible diaphragm remote from the equilibrium chamber and is adapted to move the flexible diaphragm toward and away from one opening of the second orifice passage so that the flexible diaphragm is pressed onto and retracted from the open end of the second orifice passage for closing and opening the same. Thus, the vibration damping device can change its damping characteristics, by selectively controlling the second orifice passage to be operative and inoperative for permitting and inhibiting the fluid communication therethrough.
The above type of the vibration damping device may exhibit a desired vibration damping effect based on the flows or resonance of the fluid flowing through the first and second orifice passages, with respect to the input vibrations over two different frequency bands to which the first and second orifice passages are respectively tuned. However, this type of vibration damping device may suffer from a significant deterioration of its vibration damping characteristics, especially upon application of vibrations having a frequency band higher than that the frequency band to which the second orifice passage is tuned, due to a significant increase of resistance to the fluid flow through any of the first and second orifice passages. It is therefore required a further improved vibration damping device which is capable of exhibiting an excellent damping effect over a wide frequency range, especially in a high frequency band.
To meet the above requirement, there has been proposed another type of the fluid-filled vibration damping device wherein a movable plate is supported by the partition member so as to be movable in a slight amount of distance between the pressure-receiving chamber and the equilibrium chamber. One of opposite major surfaces of the movable plate is subjected to a pressure of the fluid in the pressure-receiving chamber, while the other surface of the movable plate is subjected to a pressure of the fluid in the equilibrium chamber. Upon application of the high-frequency vibration to this type of the vibration damping device, the pressure change of the fluid in the pressure-receiving chamber may be absorbed or reduced by a displacement of the movable plate that is caused by a pressure difference between the fluid in the pressure-receiving chamber and the fluid in the equilibrium chamber.
However, the above type of the vibration damping device may suffer from a problem of difficulty in obtaining a sufficient space for accommodating the movable plate in the partition member wherein the first and second orifice passages have already been formed, resulting in an inevitable increase in the size of the partition member and a complication of the structure of the partition member. It is therefore a significant point how to arrange the movable member in the partition member with a sufficiently large size required for an effective operation thereof to absorb the pressure change in the pressure-receiving chamber, without requiring an desirable increase in size of the partition member. In particular, a degree of freedom in tuning of the first and second orifice passages, e.g., in determining the lengths or the cross-sectional areas of the first and second orifice passages, may possibly be restricted, depending upon the position of the movable plate in the partition member. In addition, the accommodation of the movable plate in the partition member is likely to restrict a freedom in determining positions of opposite openings of the second orifice passage, leading to an undesirable limitation of a freedom in determining a movable portion of the flexible diaphragm at which the flexible diaphragm is pressed onto or retracted from one of the openings of the second orifice passage. The inadequate selection of the movable portion of the flexible diaphragm due to the restriction of the above-indicated freedom in determining the movable portion, may possible cause a significant adverse influence on a durability of the flexible diaphragm and an efficiency of movement of the movable portion of the flexible diaphragm.